This invention relates generally to step wave power converters for transforming power from power sources supplying DC voltage input into AC power. More specifically, this invention relates to step wave power converters for providing greater input control over multiple DC power buses and for more accurately simulating single- or multiple-phase AC waveforms. While this invention is particularly directed to the transformation of power from DC power sources to AC power, it should be noted that AC power sources can be readily converted to DC power sources through the use of a rectifier. Therefore, the scope of this invention is not limited to strictly DC-to-AC power conversion.
Prior art patents and publications describe various single-phase step wave power converters for transforming DC voltage input into a step wave AC output. FIG. 1 is a schematic illustration showing one example of a prior art power converter. Referring to FIG. 1, one single-phase step wave power converter of the prior art uses one transformer 2 for each step of the step wave output. A single DC power source is used to supply power to each of the transformers 2 in the power converter. Each transformer 2 has three windings, including two primary windings P1, P2 and one secondary winding S. The two primary windings P1 and P2 are electrically coupled to the DC power source through four gates G1–G4. The gates G1–G4 control the flow of current through the primary windings P1, P2 in order to produce a step of the AC output from the secondary winding S. The two primary windings P1, P2 in each transformer 2 are identical to each other except that they are oppositely connected to the DC voltage source. Because of their reverse connections, they induce opposite polarity voltage in the secondary winding S. The secondary windings S of the transformers are connected together in series so that their outputs can be combined to produce the step wave AC output.
In operation, the gates G1–G4 are controlled to alternately pulse DC current through the primary windings P1, P2. Current flow through a positive polarity primary winding P1 induces a positive step output from the corresponding secondary winding S, while, conversely, the flow of current through a negative polarity primary winding P2 induces a negative step. Steps from the secondary windings S of all of the transformers 2 are added together to form the overall AC waveform. Consequently, pulsing DC current through the primary windings P1, P2 at the appropriate time intervals causes the secondary windings S to output an approximate AC waveform.
U.S. Pat. No. 5,373,433 issued to Thomas (Thomas) provides an improvement in the art with respect to single-phase power inverters operating from a single DC power source. Specifically, Thomas discloses switching bridges for controlling DC voltage input into multiple transformers from a single source. Each switching bridge includes four switches arranged in two parallel lines, each of which has two series connected switches. The switching bridges are controlled so that the transformers produce either a positive, zero, or negative output voltage step at a given time. According to Thomas, the transformers “preferably have turns ratios that are multiples of each other in order to provide both good resolution and a wide dynamic range of the [AC output] signal.” Col. 5, 11. 58–62. For example, Thomas explicitly discloses a single-phase power converter having three transformers capable of producing voltage outputs from their secondary windings of ±15V, ±45V, and ±135V, respectively. The output voltages from the secondary windings of all of the transformers are combined in series. Thomas produces a fairly accurate AC waveform by controlling timing and sequencing of the voltage contributions from the three transformers to transition sequentially through each of twenty-seven different possible overall output voltage levels. A special decoder circuit is also provided to prevent accidental shorting across the DC voltage input which would occur if two switches in a series connected pair were closed at the same time. Despite its improvements, Thomas does not contemplate either the use of multiple power sources or three-phase operation.
Another prior art topology is described in U.S. Pat. No. 5,631,820 issued to Donnelly et al. (Donnelly). Donnelly provides an improvement in the art by using three gates instead of four to control current flow through primary transformer windings. Also, although using transformers having two primary windings and one secondary winding, Donnelly's switching architecture allows each primary winding to be used to produce either a positive or a negative step, rather than only one or the other. Donnelly also provides an improvement in the art by contemplating the use of multiple power sources, but fails to provide seamless integration and management of the multiple power sources based on their performance characteristics. Donnelly also discloses a three-phase power converter topology that has nine gates and one three winding, three-phase transformer per step.
Other prior art patents and publications also describe three-phase step wave power converters for converting DC voltage from one or more DC power sources to a step wave AC output. Referring to FIG. 2, one example of a prior art three-phase step wave power converter includes multiple three-phase transformers 4, each having three windings (two primary P1, P2 and one secondary S) per phase per step. The configuration of each phase is similar to the single-phase arrangement of the prior art described above with reference to FIG. 1. Each phase of each transformer includes two primary windings P1, P2 and a secondary winding S. The two primary windings P1, P2 of each phase are identical to each other except for their opposite connections to the DC power source. Four switches G1–G4 are used to control current flow through the primary windings P1, P2 of each phase. The switches are used to alternately pulse DC voltage through the primary windings P1, P2 in order to generate steps of the AC waveform for that particular phase from a corresponding secondary winding S. The contributions output from the secondary windings S of the transformers for a given phase are combined together in series to produce the step wave AC output for that phase.
Unfortunately, this prior art configuration is bulky, requiring a three winding, three-phase transformer 4 controlled by 12 gates for each step. Also, each primary winding P1, P2 contributes only one positive or one negative step towards the overall AC waveform output and the total number of steps of the AC output directly corresponds to the number of primary windings used to produce the output. To get better resolution in this three-phase AC waveform output, therefore, more transformers must be added to the system, further increasing its bulkiness.
It should be noted that in each of the prior art three-phase step wave converters, the three-phase transformers 4 used are wye-wye transformers, meaning that both the primary P1, P2 and secondary windings S are arranged in wye configurations. This configuration is presumably used to avoid voltage contention which occurs between delta and wye connections in delta-wye transformers.
A further drawback of each of the prior art power converters is that the step wave AC output is generally blocky as a result of the mere addition of positive and/or negative block steps to form the AC waveform output. Although blocky AC waveforms are acceptable for many applications, they are less than desirable for use in many modern electronic devices such as computers, televisions, etc., which perform better and last longer when power is supplied to them using a closely regulated AC power supply.
Therefore, the industry faces several problems related to conventional step wave power conversion. First of all, the industry has been unable to seamlessly integrate power from multiple power sources based on their performance characteristics. The industry has also failed to produce a step wave AC output that more closely approximates an ideal AC waveform. Additionally, the industry has been unable to produce a three-phase step wave AC power output in a more efficient manner. The industry has further failed to enhance the resolution of the AC waveform output from a three-phase step wave power converter without increasing the number of primary transformer windings. Furthermore, the industry has not succeeded in allowing a single power source to selectively supply power to multiple transformers when other power sources become disabled or go offline. Nor has the industry succeeded in preventing backfeed to the power grid or in allowing any DC power source connected to the converter to be charged from any of the other power sources connected thereto.
Accordingly, the industry would be benefitted by a step wave power conversion method and apparatus which provides seamless integration between multiple power sources. The industry would be further benefitted by a step wave AC output which more closely approximates an ideal AC waveform. The industry is in further need of a more efficient step wave power converter. The industry would also be benefitted by a method of converting DC voltage into three-phase power output with enhanced resolution with simpler circuitry. The industry is in still further need of a step wave power converter which allows a single power source to selectively supply power to multiple transformers when other power sources become disabled. Still further needs in the industry include preventing backfeed to the power grid and allowing any DC power source with storage capability connected to the converter to be charged from any of the other power sources connected to the converter.